Atmospheric vacuum breaker (AVB)
A simple, non-testable backsiphonage prevention device that uses a float or disc to seal against flow and admit air when pressure is lost.
An atmospheric vacuum breaker is one of the simplest backflow prevention devices. It contains a floating disc or poppet that lifts and seals against an air vent while water flows, then drops open when flow stops and pressure is lost, admitting atmospheric air that breaks any developing siphon. It has no springs, no test cocks, and no shutoff valves, which makes it inexpensive and compact but also means it cannot be field tested to verify performance.
The AVB's simplicity comes with strict operating limits. It protects against backsiphonage only, never backpressure. It cannot be under continuous pressure for extended periods, because the disc can stick in the sealed position and fail to open when needed; this is why no control valve may be installed downstream of it. It must also be mounted above the highest downstream outlet. Within those limits it serves well on hose bibb connections, individual fixtures, and irrigation zones valved upstream.
Testers and property owners should understand that because an AVB is a non-testable device, its reliability depends entirely on correct application and periodic visual inspection rather than gauge testing. During surveys, check that no downstream valves have been added, that the device sits at the proper elevation, and that it is not subjected to constant pressure. Where an application cannot meet those conditions, a testable assembly such as a PVB or SVB is the appropriate substitute.
Related terms
| Pressure vacuum breaker (PVB) | A testable backflow prevention assembly with a spring-loaded check valve and an air inlet valve that protects against backsiphonage only. |
| Spill-resistant vacuum breaker (SVB) | A testable vacuum breaker that functions like a pressure vacuum breaker but is designed to avoid spilling water during pressurization, making it suitable for indoor use. |
| Backsiphonage | Backflow caused by a drop or loss of pressure in the supply piping that siphons water backward through a cross-connection. |
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